Embodiments of the present invention relate to a method of transmitting data by inductive coupling, including the steps of receiving an antenna signal using an inductive antenna circuit in the presence of an alternating external magnetic field, extracting from the antenna signal a first periodic signal, producing a second periodic signal using a synchronous oscillator having a synchronization input receiving the first periodic signal, placing the oscillator in a free oscillation mode, and applying to the antenna circuit bursts of the second periodic signal to generate an active load modulation magnetic field.
Embodiments of the present invention also relate to a device for transmitting and receiving data configured to implement this method.
In a general manner, embodiments of the present invention relate to inductive coupling communication techniques, also known as “Near Field Communications” or NFC. A communication by inductive coupling generally requires a “passive” device and an “active” device. The two devices are equipped with an antenna coil. The active device emits a magnetic field oscillating, for example, at 13.56 MHz, and sends data to the passive device by modulating the magnetic field. In the following, this magnetic field is designated as an “external magnetic field”. The passive device sends data to the active device by load modulation.
The load modulation may be passive or active. Passive load modulation includes modifying the impedance of the antenna coil of the passive device at the rate of a data-carrying load modulation signal. This impedance modulation is echoed by inductive coupling in the impedance of the antenna coil of the active device. The active device may therefore extract from its antenna signal the load modulation signal used by the passive device, and deduce therefrom the data that the passive device sent it.
Active load modulation includes transmitting, at the rate of the data carrying modulation signal, bursts of alternating magnetic field. The magnetic field bursts are perceived by the active device as a passive load modulation. This technique was proposed by the applicant in European patent EP 1 327 222 (U.S. Pat. No. 7,098,770B2), see FIGS. 4A to 4E, page 8 table 4, paragraph 074.
Active load modulation offers, with respect to passive load modulation, a greater communication distance and/or a better data transmission in a hostile environment, for example an environment perturbed by metallic masses generating Foucault (Eddy) currents. Active load modulation requires, on the other hand, excitation of the antenna coil and thus a current source, but consumes much less current than a continuous emission of a magnetic field.
An active load modulation device cannot therefore be purely passive in terms of electrical supply (a purely passive device being electrically supplied by the magnetic field emitted by the active device) but is nevertheless considered as “passive” in that it does not emit the external magnetic field necessary for the communication.
To obtain a maximum communication distance, the active load modulation also requires that the load modulation magnetic field be in phase with the external magnetic field emitted by the active device. Phase rotations between the active load modulation magnetic field and the external magnetic field may cause undesirable fluctuations in the communication distance.
European Patent EP 1 801 741 describes an active load modulation NFC device using a phase locking loop to control the phase of the load modulation magnetic field (see FIG. 19 thereof). The phase locking loop includes a Voltage-Controlled Oscillator VCO, a phase comparator, and a low pass filter supplying a control voltage to the VCO. The phase comparator receives, as a reference frequency, a first periodic signal extracted from the antenna signal induced by the external magnetic field. The phase locking loop supplies a second periodic signal of which the phase is set on that of the first periodic signal. In the data emission mode, bursts of the second periodic signal are applied to the antenna circuit to generate the magnetic field bursts.
When the device switches into the data emission mode, the first periodic signal is no longer applied to the phase comparator and a sampling circuit HLD (“Sample Hold”) maintains the control voltage applied to the VCO. The phase locking loop thus switches from a synchronous functioning mode to a free oscillation mode and remains in this functioning mode until the end of the data emission.
If it is desired that the magnetic field bursts be in phase with the external magnetic field, the phase locking loop must have a slight phase drift during the entire duration of the data emission mode, which is at least equal to the data frame emission duration. In practice, the maximal phase difference tolerated for this period is generally on the order of ¼th of the period of the magnetic field oscillating at 13.56 MHz.
As an example, an ISO 14443-A frame has a duration on the order of 25.6 ms. The frequency of the periodic signal being 13.56 MHz, the phase drift of the phase locking loop in free oscillation mode is preferably not more than 18 ns, that is ¼th of the period of the magnetic field oscillating at 13.56 MHz.
However, obtaining a stability greater than 18 ns over a duration of 25.6 ms signifies that the phase locking loop must offer an extreme precision, on the order of 0.7 ppm ((18×10−9/25.6×10−3)*106). Such a precision requires a very high quality and expensive circuitry.
It may therefore be desired to provide an apparatus that allows magnetic field bursts having little phase drift relative to the external magnetic field to be provided, without using an extremely precise and expensive circuitry.